The technique referred to as Friction Stir Welding (FSW) is known, in general, for making fast durable mechanical joints which allow forces to pass between assembled parts with an efficiency which is at least equivalent to that achieved using a conventional riveted joint.
This technique, shown schematically in FIG. 1, uses a welding device which includes at least one welding head 10, which includes a rotating pin 12, a shoulder 14, which extends to the base of the rotating pin 12 and which has a diameter which is typically equal between 2 and 2.5 times the mean diameter of this rotating pin 12.
Friction stir welding of two parts 16a, 16b involves introducing the rotating pin 12 into the two parts at the joint interface 18 between the latter until the shoulder 14 makes contact with the surface of each of the parts 16a, 16b. This introduction of the rotating pin 12 into the material making up the parts 16a, 16b, is made possible by local softening of this material as a result of the heating produced by the friction of the rotating pin 12 against the two parts 16a and 16b. The dough-like state of the material of the parts 16a, 16b around the rotating pin 12 then allows this rotating pin to move along the joint interface 18. The rotation of the rotating pin 12, as well as, if appropriate, that of the shoulder 14, causes mixing of the material in the dough-like state.
The extrusion caused by the rotating pin 12 and the forging effect produced by the shoulder thus gradually results in the formation of a weld bead. This weld bead takes the form of a new metallurgical structure common to the two materials, formed as a result of recovery-recrystallization, which thus guarantees good cohesion of the two parts 16a, 16b after cooling.
As shown schematically in FIG. 2, a counter-pressure is applied on the face of each part 16a, 16b opposite the welding head 10, using a counter-form 20, in order to counteract the pressure exerted by the rotating pin 12. Such a counter-form sometimes incorporates a cooling device which allows part of the heat generated by the friction to be removed. This, in general, improves the mechanical properties of the parts after welding. Such a cooling device takes the form, for example, of channels 21 which are incorporated in the counter-form 20 and in which a heat-transfer fluid flows.
The friction stir welding technique allows so-called “butt” welding to be carried out, as shown in FIGS. 1 and 2, in which the axis of the rotating pin 12 is locally parallel to the joint interface between the parts to be assembled.
This technique also allows so-called “transparency” welding to be carried out, in which the axis of the rotating pin 12 is locally orthogonal to the joint interface between the parts to be assembled. In this case one of the parts to be assembled is interposed between the welding head and the other part to be assembled.
The friction stir welding technique in particular exhibits the advantage of being carried out below the melting point of the constituent material of the parts to be assembled, which in particular avoids problems associated with re-solidification and which usually occur with other welding techniques.
This technique, in addition, offers the advantage of not requiring any filler materials, and of not causing any emission of polluting fumes.
Furthermore the speed at which the welding device moves along the joint interface of the parts to be assembled may reach 2 meters per minute, so that this welding technique allows parts to be assembled quickly and at reduced cost.
This welding technique in addition offers possibilities for high levels of automation.
Nevertheless, known friction stir welding processes do possess drawbacks.
The counter-form used must be manufactured to strict dimensional tolerances in order to fit against the surfaces to be assembled as closely as possible, in the case of butt welding or against one of the parts to be assembled in the case of transparency welding.
If this is not the case then the dimensions of the parts to be assembled must be adjusted beforehand, for example by means of mechanical machining, so that they match the geometry of the counter-form as closely as possible.
Furthermore level of the forces exerted by the welding head may reach several tonnes, so that the counter-form must in general be of large mass, especially when the parts to be assembled have a curved form, for example in the case of the assembly of a circumferential frame 22 onto an aircraft fuselage panel 24 as shown in longitudinal section in FIG. 3. Such a circumferential frame 22 includes a flange 26 which is substantially cylindrical in shape applied onto a fuselage panel 24, as well as a web 30 which extends substantially orthogonally to the base-plate 26.
Furthermore the mass of the counter-form will be greater and more complex to manufacture if it includes a cooling device which has to be capable of extracting significant quantities of heat.